How does reduce function work?

Question

As far as I understand, the reduce function takes a list l and a function f. Then, it calls the function f on first two elements of th

Answer 1

You could also implement factorial using reduce.

def factorial(n):
  return(reduce(lambda x,y:x*y,range(n+1)[1:]))

Answer 2

The other answers are great. I'll simply add an illustrated example that I find pretty good to understand reduce():

>>> reduce(lambda x,y: x+y, [47,11,42,13])
113

will be computed as follows:

(Source) (mirror)

Answer 3

Reduce executes the function in parameter#1 successively through the values provided by the iterator in parameter#2

print '-------------- Example: Reduce(x + y) --------------'

def add(x,y): return x+y
x = 5
y = 10

import functools
tot = functools.reduce(add, range(5, 10))
print 'reduce('+str(x)+','+str(y)+')=' ,tot

def myreduce(a,b):
    tot = 0
    for i in range(a,b):
        tot = tot+i
        print i,tot
    print 'myreduce('+str(a)+','+str(b)+')=' ,tot

myreduce(x,y)

print '-------------- Example: Reduce(x * y) --------------'

def add(x,y): return x*y
x = 5
y = 10

import functools
tot = functools.reduce(add, range(5, 10))
print 'reduce('+str(x)+','+str(y)+')=' ,tot

def myreduce(a,b):
    tot = 1
    for i in range(a,b):
        tot = tot * i
        print i,tot
    print 'myreduce('+str(a)+','+str(b)+')=' ,tot

myreduce(x,y)